Development of clean coal technology
by using the feature of oxy-fuel
combustion
Hirotatsu Watanabe
Department of mechanical and control engineering,
graduate school of science and engineering,
Tokyo Institute of Technology
4th oxy-fuel capacity building course, 2/3 September, 2012
Tokyo Institute of Technology, Japan
2
Introduction
CO2 has attracted unfavorable attention as one of
the greenhouse gases, which is the main cause of
global warming.
O2/CO2 combustion (oxy-fuel combustoin) is seen
as one of the major options for CO2 capture for future
clean technologies.
CO2
transport
Caprock
Saline
aquifer
Coal firing
plant
CO2
Fig. Schematic diagram of CCS
3
Introduction
CO2
N2
Over 95% of CO2
O2
Fig. Exhaust gas concentration
Coal
Air
O2
ASU
(Air Separation Unit)
Boiler
Flue gas
treatment
Recycled gas
CO2
capture
Fig. Oxy-fuel combustion and CCS system
Direct CO2 recovery becomes possible without
additional energy consumption.
Another approach for reducing CO2 emission is the
use of renewable fuels such as biomass
4
Introduction
Coal/Biomass
or Biomass
O2
Air
ASU
(Air Separation
Unit)
CO2
N2
Over 95% of CO2
O2
Fig. Exhaust gas concentration
Boiler
Flue gas
treatment
Recycled gas
CO2
capture
Fig. Oxy-fuel combustion and CCS system
O2/CO2 biomass combustion with CCS can be used
as a sink for CO2.
O2/CO2 coal or biomass combustion is a promising
technology for reducing CO2 emission
5
Introduction
Small scale facilities are useful for fundamental
mechanism clarification of O2/CO2 combustion
Small experimental
facilities
Demonstration plant
Reaction tube
To tar trap
Infrared furnace
Biomass
sample
Valve
Scaledown
Cylinder
of CO2
Flow
meter
Valve
Thermometric
point
Cylinder
of Ar
P
Vacuum
pump
PC
Valve
Pressure
gauge
6
Introduction
• What are differences between O2/N2 and O2/CO2
combustion ?
– Recycling process
– High CO2 concentration
• Heat transfer characteristics (Thermophysical
Properties of CO2)
• CO2 chemical reactivity
– CO2 is not inert but participates in chemical
reactions primarily through the reaction
(CO2 + H = CO + OH)
7
Introduction
By using CO2 chemical reactivity, clean coal
technology for oxy-fuel combustion is potentially
developed
Mechanism clarification through fundamental
research is required
High CO2 concentration
Minerals (Na,,)
Coal or
biomass
CO2 chemical reactivity affects
gas/solid phase reaction
Gas phase reaction
・Volatile-N → NO or N2
Solid phase reaction
・Carbonate formation (Na2CO3)
8
Introduction
• Our laboratory has used different experimental
facilities and calculation for mechanism clarification of
O2/CO2 coal and biomass combustion
– Flat flame reactor, Drop tube furnace, TGA
– Detailed chemical reaction kinetics
Reaction tube
To tar trap
Infrared furnace
Biomass
sample
Valve
Flat
flame
Cylinder
of CO2
Flow
meter
Valve
Thermometric
point
Cylinder
of Ar
Primary gas
(CH4, O2, CO2, NH3/Ar)
Fig. Flat flame reactor
P
Vacuum
pump
PC
Valve
Fig. Drop tube furnace
Pressure
gauge
Fig. Thermobalance
9
Table of contents
• Effect of CO2 on gas phase reactions
– Ultra-low NOx emission by using CO2
chemical reactivity
• Effect of CO2 on solid phase reaction
– Salt formation during biomass pyrolysis
• Summary
10
Table of contents
• Effect of CO2 on gas phase reactions
– Ultra-low NOx emission by using CO2
chemical reactivity
• Effect of CO2 on solid phase reaction
– Salt formation during biomass pyrolysis
• Summary
11
Ultra-low NOx emission
Uniform field
Low NOx
emission
Recycles gas
(Mainly CO2 including NOx)
Fig. NOx conversion ratio in O2/CO2 combustion [1]
NOx emission decreased to 1/7 owing to recycling process
when equivalent ratio is assumed to be uniform
O2/CO2 combustion has potential for reducing further NOx
emission by combined with staged combustion
[1] Liu and Okazaki, Fuel 2003
12
Ultra-low NOx emission
NOx, HCN and NH3
formation
are
inhibited in fuel-rich
region
Primary gases
Coal with
gases
Secondary
gases
Air ratio
(excess O2 ratio)
High
Fuelrich
Fuellean
Low
High conc. of CO2
Fig. Staged combustion (without recycling NOx)
The effect of high CO2 concentration on NOx
formation and reduction mechanisms under
staged combustion is discussed
13
Contents
• This research
– NOx formation and reduction mechanism in
staged O2/CO2 combustion and air combustion
were investigated.
– A flat CH4 flame doped with NH3 for fuel-N was
used, and measurements were performed.
– CHEMKIN-PRO was used to investigate a
detailed NOx reduction mechanism
14
Flat flame reactor
A large part of the fuel conversion in the combustion
process occurs in the gas phase
Flat
flame
Primary gas
Honeycomb
Quartz tube (CH4, O2, CO2, NH3/Ar)
Fig. Flat flame reactor
Flat flame is very useful for one-dimensional analysis
and mechanism clarification of gas phase reactions
15
Primary combustion
GC
Exhaust
Pump
Exhaust
26
Flow
Meter
Chemiluminescent
NOx detector
Silica-gel Cold Trap
Insulator
Quarts Tube
500
Pre-heater
Flow Controllers
CH4 CO2
O2
Air
comp- NH3
ressor (Ar)
Flat flame
Primary gas
(Air, CH4, NH3 or
O2, CO2, CH4, NH3)
Ceramic
honeycomb
Fig. Schematic diagram of experimental apparatus
NOx, HCN, NH3 emissions in air or O2/CO2 combustion were
investigated under primary combustion
16
Primary combustion
Table 1 Experimental conditions (primary combustion)
Initial O2 conc. [vol. %]
Initial NH3 conc. [vol. %-CH4]
primary
Flat
flame
Primary gas
(CH4, Air, NH3/Ar .or.
CH4, O2, CO2, NH3/Ar)
Air
Oxy-fuel
21
1.0
23
1.0
Primary combustion characteristics
are important to discuss NOx emission
in staged combustion.
HCN, NH3 (gas detector) and NOx
emission of primary combustion were
measured.
17
OH radical measurement
OH radicals are relevant with NH3 and HCN
formation and decompositon.
OH* chemiluminescence images of flat
flame was acquired by ICCD camera
Interference filter
(306.3 nm)
CCD
camera
 26
p = 0.70
Flat flame
Lens
SUS mesh
(b) Flat flame
Primary gas
(a) CCD camera with
(CH4, O2, CO2, NH3/Ar)
interference filter
Fig. Schematic diagram of OH* measurement system
18
Staged combustion
GC
Exhaust
Pump
Exhaust
26 Quarts Tube
Flow
Meter
Silica-gel Cold Trap
Chemiluminescent
NOx detector
Secondary gas
(Air or O2+CO2)
Flow Controllers
CH4 CO2
O2
500
Insulator
Air
comp- NH3
ressor (Ar)
Flat flame
Primary gas
(Air, CH4, NH3 or
O2, CO2, CH4, NH3)
30
Pre-heater
Ceramic
honeycomb
Fig. Schematic diagram of experimental apparatus
NOx reduction by staged combustion was investigated
19
Staged combustion
F
a

Air ratio
 O F 
CH 
F : flow
(excess O2   
rate

 FO

 [l min-1]
ratio) 
F
CH 

2
4
2
4
 26
Secondary gas
(Air or O2, CO2) secondary
Secondary
gas
a
primary
st
30 mm
Flat flame
Ceramic
honeycomb
Primary gas
(CH4, O2, CO2, NH3/Ar)
SUS mesh
Fig. Mixing part
Primary gas
(CH4, O2, NH3/Ar)
Fig. Flame photograph
20
Experiment conditions
Table 1 Experimental conditions (staged combustion)
Air
Initial O2 conc. [vol. %]
Initial NH3 conc. [vol. %-CH4]
21
1.0
Oxy-fuel
23
1.0
s = 1.2
s = 1.2
s = 1.2
p = 0.60
p = 0.65
p = 0.70
Secondary
Flat
Flat
gas
flame
flame
(Air or O2, CO2)
Primary gas
Primary gas
Primary gas
(CH4, Air, NH3/Ar, or
Staged combustion experiments were
CH4, O2, CO2, NH3/Ar)
Nomenclature :
 : Air ratio or O2/CH4 stoic. ratio
performed by changing primary air
ratio (excess O2 ratio)
21
Results (staged combustion)
s = 1.2
Secondary gas
(Air or O2, CO2)
40 % decreased due
to CO2 reactivity
primary
Primary gas
Fig. Experimental apparatus
Primary O2/CH4 stoic. ratio [-]
Fig. NOx CR (Conversion Ratio) [2]
[2] H. Watanabe et al.
Combustion and Flame 2011
The lowest NOx CR of O2/CO2 combustion is
lower than that of air combustion by 40 %
22
Results (primary combustion)
N-min = 0.7
N-min = 0.6
O2/CH4 stoic. ratio [-]
Fig. 1 Air-fuel combustion
O2/CH4 stoic. ratio [-]
Fig. 2 O2/CO2 combustion
HCN and NH3 concentrations in O2/CO2 combustion are
quite low compared with those of air combustion
[2] H. Watanabe et al. Combustion and Flame 2011
23
Results
x
10
0
(b) O2/CO2
(a) Air
combustion
combustion
Fig. Measured OH* images at
O2/CH4 stoich. ratio stoich. of 0.7
OH* concentration is
higher
in
O2/CO2
combustion than in air
combustion
Fig. Measured profiles of
OH* chemiluminescence at
O2/CH4 ratio of 0.7
The following reaction
is
progressed
in
O2/CO2 combustion
CO2 + H → CO + OH
24
Reactions related with OH
• Reaction paths relevant with NH3 and OH
– NH3 + OH → NH2 + H2O
– NH2 + OH → NH + H2O
– NH + NO → N2 + OH
• Reaction paths relevant with HCN and OH
– HCN + OH → HNCO + H
– HNCO + H → NH2 + CO
OH radicals progress the decomposition of
NH3 and HCN
25
CHEMKIN-PRO calculation
Primary
gas
Mesh
-20
One-dimensional
plug flow reactor
Flat flame
50 [mm]
0
Temperature [K]
1800
1800
NOx formation mechanisms
in primary combustion were
investigated with detailed
chemical reaction kinetics
(GRI-Mech 3.0)
Air
Air
Oxy
Oxy
1600
1600
1400
1400
1200
1200
1000
1000
800
800
-20 -10
-10 00
-20
10
10
20
20
30
30
40
40
50
50
Distance from mesh [mm]
Fig. Temperature distribution
Table Calculation conditions
O2/CH4 stoich. ratio [-]
NH3 conc. [vol. %-CH4]
0.7
1.0
26
Calculation results (OH)
Mesh
Primary
gas
-20
Flat flame
0
50 [mm]
Calculation also shows that
OH radicals in O2/CO2
combustion is higher than
in air combustion because
of the reaction:
CO2 + H → CO + OH
Fig. Predicted OH concentration distribution
in O2/CO2 combustion
27
Results (primary combustion)
(a) Experiment
(b) Calculation
Fig. Exhaust NH3, HCN, NO concentration ( = 0.7) [2]
NH3 and HCN are decomposed in O2/CO2
combustion due to OH radical
[2] H. Watanabe et al. Combustion and Flame 2011
28
0.8
0.6
0.4
0.2
0
Exp.
(NO+HCN+NH3)exhaust / NH3,inlet [-]
(NO+HCN+NH3)exhaust / NH3,inlet [-]
Results (primary combustion)
0.8
Calc.
0.6
0.4
0.2
0
Oxy
Air
Oxy
Air
Fig. The sum of exhaust NO, HCN and NH3 concentration
Total nitrogen-compounds emission (NO, HCN, NH3) of
O2/CO2 combustion is lower than that of air combustion
N2 is easily formed during O2/CO2 combustion
29
Reaction pathways
Calculation shows reaction pathways from NH3 to N2.
NH formation by OH radical is important in N2 formation
Fig. Reaction pathways at primary = 0.6, total = 0.8 (XCO2,inlet = 0.70) [3]
[3] H. Watanabe et al. Energy and Fuels 2012
30
Effect of OH radical
HNO +OH
+O
+OH
NH3
+OH
+O
NH2
+H
+OH
NO
NH
+NO
N 2O
+OH
+H
N
+NO
+H
+NO
N2
OH radical oxidizes NH,
and converts to HNO
OH radical contributes
to NO formation when
NH3 does not remain
Fig. Reaction paths related with N2 formation
NH3
NH2
+OH
+O
Under fuel-rich condition
+OH
OH radical produces NH,
HNO
NO
which is NO reduction
+O
+OH
+OH
agency
+H
N
NH
+H
+OH
+NO
+NO
+NO
OH radical contributes
N2
N 2O
to N2 formation when an
+H
Fig. Reaction paths related with N2 formation amount of NH3 remains
31
Conclusion (ultra-low NOx)
NOx formation mechanism in O2/CO2 and air combustion
was investigated experimentally and numerically.
CO2 chemical reactivity produced OH radical through
(CO2 + H → CO + OH)
The lowest NOx conversion ratio in O2/CO2 staged
combustion was lower than it in air staged combustion by
40 % due to CO2 reactivity
Reaction pathways from NH3 to N2 were revealed, and it
was shown that OH radical contributed to N2 formation
when an amount of NH3 remains
32
Table of contents
• Effect of CO2 on gas phase reactions
– Ultra-low NOx emission by using CO2
chemical reactivity
• Effect of CO2 on solid phase reaction
– Salt formation during biomass pyrolysis
• Summary
33
Introduction
• Pyrolysis of solid fuel
– Pyrolysis occurs as the first step in solid
combustion.
– Pyrolysis has been generally investigated under an
inert gas such as N2, Ar, He.
– Understanding pyrolysis under CO2 is important for
the design of O2/CO2 biomass combustors
Gas
Solid fuel
Tar
Char
Fig. Pyrolysis
34
Introduction
• What is pyrolysis difference between under inert gas
and CO2 ?
– Heat transfer characteristics (Themophysical
properties of CO2)
– Reaction of CO2 with minerals (Ca, K, Na, Mg)
• Salt (carbonate) formation is expected.
CO2 atomosphere
Heat
Na, K，Ca
Biomass ex.) Na2CO3
Gas
Solid fuel
Tar
Char
Fig. Pyrolysis
35
Introduction
• This work
– Effect of CO2 on pyrolysis process through
mineral reactions is studied.
– Cellulose and lignin which are the main
components of biomass are heated under CO2, or
Argon atmosphere
– Metal-depleted lignin is also used to investigate
the effect of CO2 on minerals in lignin
– The
chemical
composition
of
char
is
characterized by FT-IR (Fourier Transform
Infrared Spectroscopy).
36
Experiment
Table 1 Ultimate analysis of sample (wt%)
Sample
C
H
N
S
Ash
Cellulose
44.4 6.3
0.0
0.0
0.0
Lignin (Alkali lignin)
46.5 4.5
0.1
2.5
18.2
Metal depleted lignin
53.0 5.3
0.1
4.5
3.2
When the metal-depleted lignin was prepared, metal
was removed from the lignin by stirring a mixture of
lignin, water, and ion-exchange resin
Lignin
stirring
Separate, dry
Ion-exchange process Metal depleted
resin
lignin
37
Experiment
Table 2 Ash composition [mg/g]
Sample
Lignin (Alkali lignin)
Metal depleted lignin
Na
55.0
1.03
K
10.5
0.25
Ash is removed
by 80 % by ionexchange
Lignin and metal depleted lignin were analyzed by
using FT-IR
No significant difference was observed between
both samples except for -OH:
Ion exchange
-ONa
The difference is mineral content
-OH
38
Experiment
Reaction tube
To tar trap
Infrared furnace
Biomass
sample
Valve
Cylinder
of CO2
Flow
meter
Table Experiment
conditions
Surrounding
gas CO or Ar
2
Gas flow rate
[l min-1]
0.8
Heating rate
[K s-1]
1, 10, 60
Valve
Thermometric
point
Cylinder
of Ar
P
Vacuum
pump
PC
Valve
Pressure
gauge
Fig. Schematic diagram of thermobalance
Thermogravimetric curve
is measured.
The surface chemistry of
the char was investigated
by FTIR
39
Cellulose
(Ash: 0 wt%)
Temperature [K]
Fig. Pyrolysis curves of cellulose
(Ash: 0 wt%, Heating rate : 1 Ks-1)
[4] H. Watanabe et al.
Proc. Combust. Inst. 2012, in press
Weight fraction [mg/mg d.a.f.]
Weight fraction [mg/mg d.a.f.]
Results
Char-CO2
reaction
(> 1100 K)
Lignin
(Ash: 18.2 wt%)
Temperature [K]
Fig. Pyrolysis curves of lignin
(Ash: 18.2 wt%, Heating rate : 1 Ks-1)
Contrary to expectations, the weight of the lignin chars
formed under CO2 increased by about 10 % above 873 K
40
Weight fraction [mg/mg d.a.f.]
Results
Why
increased ?
Lignin
(Ash: 18.2 wt%)
Temperature [K]
Fig. Pyrolysis curves of lignin
(Ash: 18.2 wt%, Heating rate : 1 Ks-1)
CO2 physical adsorption
Carbonate formation
have the potential to cause
an increase in the weight
fraction due to CO2
Char weight does not
change by a degassing
procedure (5 kPa for 1 h).
CO2 physical adsorption is
insignificant
Carbonate is expected to
be formed during pyrolysis
41
Results
Sample weight
Weight
=
fraction
Initial weight of CaO
Minerals can react with
CO2, and salt form.
(Ⅰ) CaO + CO2 → CaO・CO2
(Ⅱ) CaO・CO2 → CaCO3
(Ⅲ) CaCO3 → CaO + CO2
Weight fraction [-]
0.3
0.2
CaO heating
under CO2
(Ⅲ)
(Ⅰ+Ⅱ)
0.1
(Ⅰ)
0
There is a possibility that
Na or K in lignin reacts
with CO2, and Na2CO3 and
K2CO3 are formed
350
600
850 1100 1350
Temperature [K]
Fig. Thermogravimetric curves of
CaO (Heating rate : 1 Ks-1)
42
Weight fraction [mg/mg d.a.f.]
Results
Lignin
(Ash: 18.2 wt%)
Temperature [K]
[4] H. Watanabe et al.
Proc. Combust. Inst. 2012, in press
Metal-depleted
Lignin
(Ash: 3.2 wt%)
Temperature [K]
Fig. Pyrolysis curves (Heating rate : 1 Ks-1)
Mineral components in lignin react with CO2, and
carbonate is expected to be formed
43
Weight fraction [mg/mg d.a.f.]
FT-IR analysis
Lignin
(Ash: 18.2 wt%)
Temperature [K]
Fig. Pyrolysis curves of lignin
(Ash: 18.2 wt%, Heating rate : 1 Ks-1)
Surface chemistry of
chars derived under CO2
or Ar at 1073 K were
investigated by FTIR to
investigate
carbonate
formation.
Na2CO3
was
also
characterized by FTIR as
reference
FT-IR
].
u
.
a
[
y
ti
s
n
e
t
n
I
].
u
.
a
[
y
ti
s
n
e
t
n
I
1450 cm-1
Aromatic C-C
Carbonate
44
880 cm-1
Carbonate
Na2CO3
Char derived
from
Ar lignin
(at
1073 K)
CO2
Ar
CO2
2500 2300 2100 1900 1700 1500 1300 1100
Wave number [cm-1]
900
700
Fig. FTIR spectra of Na2CO3 and char derived from lignin
FT-IR
Table The atomic group and structures
1450 cm-1
Ar
].
u
.
a
[
y
ti
s
n
e
t
n
I
45
[4] H. Watanabe et al.
Proc. Combust. Inst. 2012, in press
CO2
Wave number Atomic group
(cm-1)
and structures
1450
Aromatic C-C
Carbonate
Ar
Peak area corresponding to
carbonate under CO2 is
almost twice of that under Ar
CO2
1600 1500 1400 1300
Wave number [cm-1]
Fig. FTIR spectra of chars
focusing on 1450 cm-1
A salt such as Na2CO3 or
K2CO3 is formed during
lignin pyrolysis under CO2
FT-IR
The difference of peaks for
C=O appeared (1730 cm-1).
1730 cm-1
].
u
.
a
[
y
ti
s
n
e
t
n
I
].
u
.
a
[
y
ti
s
n
e
t
n
I
840 cm-1
Na2CO3
Char derived
from
Ar lignin
(1073 K)
CO2
Ar
CO2
2500 2300 2100 1900 1700 1500 1300 1100
Wave number [cm-1]
900
700
Fig. FTIR spectra of char derived from lignin and Na2CO3
46
Results
Ar
].
u
.
a
[
yti
s
n
e
t
n
I
Table The atomic structures
Wave number Atomic group
(cm-1)
and structures
1770-1600
Over 1700
47
C=O
C C
O
CO2
Below 1700
C
O
Ar
CO2
1850 1750 1650 1550
Wave number [cm-1]
Fig. FTIR of spectra focusing
on C=O group (Char, 1073 K)
C=O groups which are not in
conjunction with aromatic ring
are only found in chars
formed under CO2
[4] H. Watanabe et al.
Proc. Combust. Inst. 2012, in press
48
Char yield [mg/mg d.a.f.]
Heating rate
0.8
12 %
8%
0.6
Ar
CO2
7%
0.4
0.2
1 Ks-1
10 Ks-1 60 Ks-1
Fig. Char yield derived
from lignin of 1073 K
Although, an increase in
char yield under CO2
declines with increasing
heating rate, carbonate is
formed at various heating
rate
49
Mechanism
• Alkaline compounds highly favor the carbonization,
dehydration, decarboxylation, and demethoxylation
reactions, leading to a modified carboneous
structures.
• Sodium ion is very small and it can penetrate into
the biomass textures and break intermolecular
hydrogen bridges under heating.
• Breaking the hydrogen bridges by carbonate
compound seems to form C=O group not
associated with an aromatic ring.
• Futher investigations are required to clarify more
detailed catalytic mechanisms
50
Conclusion (carbonate)
In this study, the effect of CO2 on pyrolysis was
investigated. Cellulose, lignin, and metal-depleted lignin
pyrolysis experiment were performed.
•Pyrolysis of lignin, but not that of cellulose and metaldepleted lignin, was affected by CO2.
•The salts such as Na2CO3 or K2CO3 were formed
during lignin pyrolysis under CO2
•It was suggested that these salts affected the char
formation reaction, in that, char formed during lignin
pyrolysis under CO2 had unique chemical bands
51
Table of contents
• Effect of CO2 on gas phase reactions
– Ultra-low NOx emission by using CO2
chemical reactivity
• Effect of CO2 on solid phase reaction
– Salt formation during biomass pyrolysis
• Summary
52
Summary
• In this presentation, unique CO2 characteristics
such as OH radical and carbonate formation
were presented.
• Under specific condition, OH radical formed by
CO2 reactivity can be used for low-NOx emission.
• Carbonate was found during lignin pyrolysis
under CO2, while it was not found in air
combustion.
Thank you very much
for your kind attention